Database 触发器作为完整性的判断约束

我必须修复由EntityFramework4自动创建的旧MS SQL数据库中的一些问题.让我们以下表为例:

CREATE TABLE a (
 id int NOT NULL PRIMARY KEY,
 date datetime NOT NULL,
 client_id int NOT NULL //references to clients table
 -- other properties related to entity a
);

这是一种抽象的实体，实际上我有8个子表来定义类型，其中包括一些其他属性.让我们以此为例:

CREATE TABLE b (
 id int NOT NULL PRIMARY FOREIGN KEY REFERENCES a (id)
 -- other properties related to entity b
);

CREATE TABLE c (
 id int NOT NULL PRIMARY FOREIGN KEY REFERENCES a (id)
 -- other properties related to entity c
);

也就是说，一个客户每天只能有一个b或c(或两者)，理想的约束是UNIQUE，但由于属性位于不同的表中，这是不可能的(根据我的知识).我采用的解决方案是创建触发器，判断每个子类型的唯一性:

CREATE TRIGGER trigger_b_upsert
ON b
AFTER INSERT, UPDATE
AS  
    IF EXISTS (
        SELECT 1
        FROM a a
        JOIN b b ON a.id = b.id
        WHERE a.client_id IN (
            SELECT client_id
            FROM a a
            JOIN inserted i ON a.id = i.id
        ) AND date IN (
            SELECT date
            FROM a a
            JOIN inserted i ON a.id = i.id
        )
        GROUP BY date, client_id
        HAVING count(*) > 1
    
    )
   BEGIN
      RAISERROR ('not allowed.', 10, 1)
      ROLLBACK TRANSACTION
   END

c个也一样.

INSERT INTO a (id, date, client_id) VALUES (1, '2050-01-01', 1);
INSERT INTO b (id) VALUES (1);
INSERT INTO a (id, date, client_id) VALUES (2, '2050-01-01', 1);
INSERT INTO b (id) VALUES (2); -- should raise error

INSERT INTO a (id, date, client_id) VALUES (1, '2050-01-01', 1);
INSERT INTO b (id) VALUES (1);
INSERT INTO a (id, date, client_id) VALUES (2, '2050-01-01', 1);
INSERT INTO c (id) VALUES (2); -- should work

INSERT INTO a (id, date, client_id) VALUES (1, '2050-01-01', 1);
INSERT INTO a (id, date, client_id) VALUES (2, '2050-01-02', 1);
INSERT INTO a (id, date, client_id) VALUES (3, '2050-01-01', 1);
INSERT INTO a (id, date, client_id) VALUES (4, '2050-01-02', 1);
INSERT INTO b (id) VALUES (1);
INSERT INTO b (id) VALUES (2); -- should work
INSERT INTO c (id) VALUES (3);
INSERT INTO c (id) VALUES (4); -- should work

因为我对MS SQL不是很熟悉，所以我想知道这种方法是否正确，以及是否存在一些性能问题/改进.

Edit:
@GarethD has shown a brillant solution to my problem, but I have another case of trigger I need to improve, if possible. Let's take the same table a and define two new ones:

-- another table, different from a, which as well references the client
CREATE TABLE e (
 id int NOT NULL PRIMARY KEY,
 client_id int NOT NULL //references to clients table
 -- other properties related to entity e
);

CREATE TABLE d (
 id int NOT NULL PRIMARY FOREIGN KEY REFERENCES a (id)
 e_id int NOT NULL PRIMARY FOREIGN KEY REFERENCES e (id)
 -- other properties related to entity d
);

如您所见，该 struct 允许在由e连接的a、d中插入一条记录，其中a.client_id可能不同于e.client_id.必须否认这种情况.我定义了一个简单的触发器:

CREATE TRIGGER trigger_d_upsert
ON d
AFTER INSERT, UPDATE
AS     
    IF EXISTS (
        SELECT 1
        FROM a a
        JOIN inserted i ON a.id = i.id
        JOIN e e on e.id = i.e_id
        WHERE e.client_id != a.client_id
    
    )
   BEGIN
      RAISERROR ('Conflict client_id.', 10, 1)
      ROLLBACK TRANSACTION
   END 

有什么更好的解决方案的建议吗？